A previously described microfluidic electrochemical cell [M. J. Willey and A. C. West, Electrochem. Solid-State Lett., 9, E17 (2006)] has been used to characterize the rates of adsorption and desorption of polyethylene glycol (PEG) onto copper (Cu) during electrodeposition. Galvanostatic and potentiostatic response to the introduction or removal of PEG from the bulk solution is measured. Adsorption time constants are similar to 0.5 s or less for bulk PEG concentrations greater than 50 ppm. For lower concentrations, adsorption is mass-transfer controlled for the conditions prevailing within the microfluidic cell, where the average diffusion layer thickness is similar to 2 mu m. PEG desorption rates are much slower, typically between 10 and 100 s. For most conditions, the electrochemical signal during desorption consists of two regions, where the first stage shows a slow linear variation and the second stage a fast exponential-type transition. In the first stage, it is hypothesized that the PEG layer blankets the electrode surface. Once the polymer-layer thins sufficiently, the layer becomes patchy, and the second desorption stage commences. (c) 2006 The Electrochemical Society.